The fibrillation of synthetic polymer films is an operation that has been known for quite some time and in general has been used in the manufacture of ropes, strings, textile staples, as well as for obtaining flat reticular structures suited for use as reinforcement (stiffening) of laminated materials such as, for instance, paper, cement slabs, non-woven fabrics, etc.
Among the various methods for preparing the above mentioned structures, the most important consist in embossing or slitting mono-oriented films by means of heated points or blades, and by then stretching or extending the film thus obtained crosswise (transversally), thereby imparting a netlike structure to it.
Such methods are described in U.S. Pat. No. 3,137,746 and in British Pat. No. 1,083,847.
According to British Pat. No. 1,337,442 the crosswise stretching operation is made to follow the superimposition or overlapping of the single nets at a certain angle to each other, in its turn followed by a lamination of the assembly of superimposed nets in order to obtain one single composite net-shaped structure.
Since the crosswise stretching of the fibrilled film is a difficult and delicate operation, the continuous preparation of such composite structures requires at least two distinct operations, the first being the stretching and the second being a superimposition, carried out with lines of machines in an angled position to each other. The extreme lightness and delicacy of the material, the phenomenon of longitudinal shrinkage of the film during the crosswise extension, as well as the poor resistance of the fibrils to longitudinal slip stresses are all elements that make the stretch-reticulation operation quite critical and thus hinder, in this case, the use of the conventional stretching or opening systems used in the textile art, such as clamps or pins supporting chains or other rigid retractor elements of like kind.
On the other hand, the fibrilled film opening systems already suggested in the prior art, and which consist, for instance, according to French Pat. No. 1,331,095, in compressing the film between elastic bands while the bands are forced to expand transversally, have limits and present difficulties with regard to the achievement of micro-fibrilled structures, as well as with regard to the application of the method to the continuous preparation of composite structures with a plurality of mutually angled layers of nets.